An optical control element, such as an optical modulator, is an essential device in optical communication systems, optical measurement equipment, and optical signal processing. In the conventional long-haul optical communications, the baseband transmission systems were mainly used, broadband operations of optical modulators are required.
In current systems, an optical modulator having an optical waveguide for confining and controlling propagating light and a traveling wave-type electrode for applying modulation signals (electric signals) on a lithium niobate (LiNbO3) substrate is mainly used.
On the other hand, in an optical modulator with resonant-type electrodes, a resonant standing wave of the electric signal is generated in the electrode, when electric signal is fed at a specific frequency. Since a resonant type optical modulator is based on the resonance of the electric signal as mentioned above, it operates efficiently especially when electric signal of the specific frequency is fed. The modulation efficiency per unit length of the electrode is better than in the traveling wave-type optical modulator. However, since a resonant-type modulator is a band operation modulator that operates only in a specific band, the modulator is not used in baseband transmutation systems, is used as optical modulator for an optical clock generators, optical broadcasting systems, and the like.
In next-generation optical communication systems, an optical modulator that can operate at extremely high speeds more than 40 Gbps, is required; however, in such a high frequency range, signal losses increase on a traveling electrode is a defect. Therefore, so-called resonant-type electrode on which resonant standing waves are induced appears promising electrode.
In addition, optical modulators having a plurality of resonant-type electrodes placed along an optical waveguide have been also proposed. As examples of these modulators, optical modulators having resonant-type electrodes of same resonant frequency are placed as shown in Patent Document 1 and Non Patent Documents 1 and 2, and a transmitting system of multiple signals of CATV, satellite broadcasting by modulating separately using cascaded external modulators located in series (generally called “cascaded integration”) as shown in Patent Document 2 and Non Patent Documents 3 and 4, have proposed.
However, when a plurality of optical modulating sections is used to compose an optical control device, such as an optical modulator, it is necessary to apply the modulation signals corresponding to the respective modulating sections, and there is a problem in that a modulation signal generator circuit for the operation becomes intricate. Furthermore, in order to generate a proper modulated light signal, it is necessary to adjust a modulation timing between electric signal and propagating light through each of the modulating sections, and therefore means for adjusting the timing of the modulation signal applied to each modulating sections are required, and there is a problem in that an entire apparatus including the optical modulator becomes complicated, and the costs are increased.